1. Field of the Invention
The present invention relates to a gas analyzing method and a gas analyzing apparatus that perform qualitative analysis as a technique to determine component gas contained in mixed gas.
2. Description of the Related Art
There has been known a technique for qualitative analysis, which utilizes mass analysis process based on Electro-Impact Ionization (hereinafter referred to as EI) and a library searching system. This method ionizes a sample gas through EI process, separates ions for each mass-to-charge ratio (m/z) to measure ion intensities thereof, and thus obtains mass spectrum. The mass spectrum is a graph with abscissa axis being m/z (mass-to-charge ratio) and ordinate axis being ion intensity. Hereinafter, measurement data of mass spectrums obtained based on EI may be referred to as EI measurement data.
The obtained mass spectrum is identified by using a library searching system which includes a database for reference (also referred to as a reference database), such as NIST (National Institute of Standards and Technology), that is, a material name is determined based on the obtained mass spectrum. Specifically, each of a plurality of mass spectrum data stored within the reference database and EI measurement data are compared with each other through a profile fitting, to thereby determine its material name.
The EI process uses relatively high energy, such as 70 eV, for ionizing gas. Therefore, the EI process does not choose specific kinds of gas to be ionized, that is, it is applicable to nearly all kinds of volatile compounds. In the EI process, when high energy of 70 eV is supplied to component molecules of gas, fragment ions generated due to fragmentation of the parent molecules contained in gas are observed as well as ions (i.e., parent ions) generated correspondingly to parent molecules. The fragment ions yield an important factor in estimating the structure of molecules which is forming the gas. However, a plurality of mass spectrums stored in the reference database corresponds to gases having single component, respectively. In the case that the gas being the target of qualitative analysis is a mixed gas composed of a plurality of component molecules, there is a problem that, when using the library searching system, the generation of the fragment ion results in acting as interfering ion on the contrary and thus makes it difficult to identify components, and eventually a highly accurate qualitative analysis cannot be made.
In order to realize analysis of such a mixed gas, there has been conventionally proposed a qualitative analysis through, for example, a gas chromatography mass analysis method (GC/MS), as illustrated in FIG. 14. In the method, the mixed gas is collected by a trap and then decomposed into a plurality of, for example, three: A, B and C component gases by means of a gas chromatograph (GC) on time-series basis. Subsequently, component gases are introduced into a mass analysis device (MS) sequentially. The reason for collecting gases by the trap is that the next gas sample cannot be introduced within the period of time (a few minutes to ten and a few minutes) during which the separation by the GC is performed.
In the mass analysis device (MS), respective steps of the EI process, ion separation, and ion intensity measurement are executed, and then mass spectrums of decomposed component gases A, B and C are measured, respectively. An operation processing device provided at a latter part of the mass analysis device (MS), for example, is configured to include a computer, and performs control as illustrated in FIG. 15. That is, the operation processing device stores EI measurement data (i.e., mass spectrums) of respective gas components A, B and C in a memory and may select one from among them. Then, the operation processing device makes comparison between selected EI measurement data and reference data (i.e., mass spectrums) read out from the NIST database in terms of m/z (mass-to-charge ratio) and ion intensity, determines substance names for the EI measurement data, and stores them in the memory. The device performs the processing for other component gases B, C too, and finally identifies component gases contained in the mixed gas from the substance names stored within the memory.
According to the aforementioned GC/MS, since a fragment ions derived from one parent ion never overlaps fragment ion derived from another parent ion, the GC/MS is capable of correctly identifying component gases. However, if a mixed gas is, for example, an unknown component sample evolved from a sample, there is a problem that evolved gas cannot be analyzed in real time, because gases must be collected by means of the trap. In the GC/MS, there may also be another obstacle for analysis due to heat instability or the like of unstable component gases including radicals.
It is to be noted that, “real time analysis” means:                directly analyzing a gas evolving from gas source such as a sample, etc., the gas changing its components, ratio of components (i.e., concentration) or the like with the lapse of time;        analyzing it simultaneously or instantaneously;        analyzing it without once collecting it;        analyzing it without adding secondary process; or        analyzing it in a state maintaining component content and ratio of components when it has been evolved.        
There have been known ionizations, such as chemical ionization, photo-ionization, etc., that can suppress occurrence of fragment ions, which may be a disadvantage of EI. These ionizations are sometimes called soft ionization since they can suppress occurrence of fragment ions. According to the soft ionization, since only component molecules of gas can be selectively ionized and observed, each gas component contained in a mixed gas can be identified in real time for each molecule ion. However, in a mass analysis based on the soft ionization, what can be obtained is only molecule ion information, so that there is a problem that components having the same mass ions cannot be discriminated from each other. Further, results of the mass analysis based on the soft ionization do not have structure information given by fragment ions, so that there is also a problem that accurate qualitative analysis cannot be always made.
The present inventors have already proposed a mass analysis device that has both functions of EI process and soft ionization process. Such a mass analysis device is disclosed in, for example, PCT Publication WO2007/108211 pamphlet (page 20-48, FIG. 1). The PCT Publication WO2007/108211 describes in greater detail a technique of how to independently carry out each of the EI process and soft ionization process, and how to carry out them simultaneously. In addition, it briefly teaches advantage of obtaining the both of EI measurement data and soft ionization measurement data. However, it does not teach the use of the EI measurement data and the soft ionization measurement data for qualitative analysis of mixed gas and other gas analyzing techniques.
As described above, in a method based on the EI process and the library searching, there is a problem that qualitative analysis cannot be conducted with sufficient accuracy due to overlap of fragment ions, in the case that the analysis target is a mixed gas. Additionally, in the measurements based on the soft ionization process such as PI process, since identification of component molecules cannot be made in a region where ion peaks overlap, all the same, qualitative analysis of mixed gas cannot be conducted with sufficient accuracy. Also, in the qualitative analysis of mixed gas through GC/MS, although accuracy of the qualitative analysis is enhanced, there is a problem that real time analysis cannot be conducted.